1. Technical Field
The present disclosure relates to a filtering device and an environment recognition system which determine, when calculating a difference value (parallax) of an object among multiple comparison targets, whether the difference value is valid.
2. Related Art
There are conventionally known a technique, such as collision avoidance control, which detects specific objects including another vehicle located ahead of a vehicle, and avoids a collision with a leading vehicle, and a technique, such as a cruise control, which controls so as to maintain an inter-vehicle distance with a leading vehicle at a safe distance (for instance, see Japanese Patent (JP-B) No. 3,349,060).
Such a collision-avoidance control and cruise control derive a parallax by using so-called pattern matching, in order to acquire a relative distance from the vehicle, of an object located ahead the vehicle. The pattern matching acquires image data from, for example, each of two imaging devices of which viewpoints differ from each other. The pattern matching then extracts any one of blocks (hereinafter, referred to as “the reference block) from an image (hereinafter, referred to as “the reference image”) based on the image data generated by one of the imaging devices, and then searches a highly-correlated block (hereinafter, referred to as “the comparison block) from an image (hereinafter, referred to as “the comparison image”) based on the image data generated by the other imaging device. Then, the pattern matching refers to imaging parameters, such as installed positions and focal lengths of the imaging devices, uses so-called a stereo method or a triangulation method to calculate relative distances of the object with respect to the imaging devices based on the derived parallax, and converts the calculated relative distances into three-dimensional (3D) positional information which contains a horizontal distance and a height of the object in addition to the calculated relative distances. Further, various recognition processing are performed using the 3D positional information. Note that the term “horizontal” as used herein refers to screen transverse or lateral directions, and the term “vertical (described later)” as used herein refers to screen vertical directions which are perpendicular to the horizontal directions.
The pattern matching calculates a correlation of a block in the comparison image with each block in the reference image, while horizontally shifting the block in the comparison image, and uses a difference (difference value) in coordinates between the comparison image of the most-correlated block and the corresponding block in the reference image, as the parallax. However, when objects having similar features are located horizontally or subject parts having similar features are located horizontally in one object, one feature may be matched with other similar objects or subject parts, leading to an erroneous derivation of the parallax.
Therefore, for example, JP-B No. 3,287,465 discloses another pattern matching technique that, when one reference block in the comparison image has the highest correlation with multiple reference blocks in the reference image, determines only a parallax regarding the reference block having the minimum parallax to be valid.
However, for example, in a case where three or more similar objects or subject parts continue, if one object in the reference image is matched with another object or subject part in the comparison image located apart from the position of the object in the reference image, the derived parallax (difference value) will be greatly different from a correct parallax, and there may be a possibility that an object located away or deeper from another object located closer to or toward the vehicle is erroneously determined to have an opposite or reversed positional relation. In such a case, even if the technique of JP-B No. 3,287,465 is adopted, still the same results will be obtained as well if the erroneously-derived parallax is once determined to be valid. Therefore, in such a case where similar objects or similar subject parts continue, the parallax which should be invalidated cannot be effectively excluded.